Sorry to hear this. You can try calling VNA and see what they say. The big issue is that you have no documentation (VNA sends you the CPO warranty document a few weeks after the sale) that you accepted the CPO with the car. It's your word against their word, unfortunately. Usually you sign stuff with the finance person and not the sales manager. That's when they try to sell you all kinds of other insurances and warranties like VIP and LowJack, etc..
Again, try VNA but you probably won't get far with this unfortunately. But since you're selling the car, just sell it as is. You'll get the fair going price. Not everyone shops based on whether the car is CPO or not. If it's been serviced and well-maintained, that will be the selling point over a CPO.
If you are trying to get a major repair covered under what you thought was a CPO warranty then that's a different story. But you're selling it, so no big deal really.
"and the price was higher than private sellers".....the price will always be higher from a dealer than a private sale, CPO or not......